Composition for electrolytic descaling of titanium and its alloys

ABSTRACT

A non-aqueous electrolytic bath for descaling titanium and its alloys utilizes a hydrofluoric acid bearing compound such as 48 50 percent hydrofluoric acid to provide the attack ion in a glacial acetic acid carrier. A buffering agent such as sulfuric acid, acetic anhydride or sulfur trioxide acts as a dehydrating agent picking up water from the hydrofluoric acid to deionize the same and prevent attack on the titanium.

D United States Patent 1 [111 3,725,224 Kendall [451 Apr. 3, 1973 [54]COMPOSITION FOR ELECTROLYTIC 2,780,594 2 1957 Dailey .204 141 DESCALING0F TITANIUM AND ITS 3,030,286 4/1962 Tao .204/141 ALLOYS FOREIGN PATENTSOR APPLICATIONS a '1' [75 Inventor W Kendall Bomta Cd If 593,403 3/1960Canada ..204 141 2047 19 [731 Assignee: Rohr Industries. Inc., ChulaVista, 38/ 4/ 63 Japan 204,141

Calif. OTHER PUBLICATIONS Rohm & Haas Co. Brochure SP-315 November 1964[22] Filed: June 30,1971

1 Appl' 5 11 Primary Examiner-John H. Mack Assistant Examinep-T.Tufariello Related US. Application Data Attorney-George E. Pearson [63]Continuation-impart of Ser. No. 816,460, Feb. 28,

1969, abandoned. [57] ABSTRACT A non-aqueous electrolytic bath fordescaling titanium [52] US. Cl. ..204/l4l.5, 134/3, 134/41, and itsalloys utilizes a hydrofluoric acid bearing [51] I t Cl C23 1,02 C23)1,00 pound such as 48 50 percent hydrofluoric acid to n g d th ttack n a1 ial ti 'd arri provi e e a 10 111 g ac ace c act 0 er [58] Field ofSearch ..204/14l.5, 252/79.4,134/3, A buffering agent such as sulfuricacid acetic 134/41 hydride or sulfur trioxide acts as a dehydratingagent picking up water from the hydrofluoric acid to [56] Referencescued deionize the same and prevent attack on the titanium.

UNITED STATES PATENTS 9 Claims No Drawings 7 9 3,616,279 10/1971 Kendall..204/141 3,068,158 12/1962 Griiss ..204/l41 3,002,899 10/1961 Reid..204/141 COMPOSITION FOR ELECTROLYTIC DESCALING OF TITANIUM AND ITSALLOYS CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of patent application for Composition forElectrolytic Descaling of Titanium and Its Alloys by Earl W. Kendall,Ser. No. 816,460, filed Feb. 28, 1969 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to electrolyticmethods and solutions for descaling titanium and its alloys and is alsoeffective in use on ferrous alloys including metals such as nickel,chromium, cobalt, tungsten, vanadium and molybdenum. Articles formed ofthese metals and alloys generally are heat treated to facilitate formingand to bring out and develop maximum mechanical properties andcharacteristics required in the end use of the articles. As a result ofsuch heat treatment, refractory pyrolytic oxides (scale) are formed onthe surfaces of the articles. These pyrolytic refractory oxides areinsoluble in acids and are therefore extremely difficult to remove by achemical immersion process.

Heretofore it has been the practice, in accordance with certain priorart descaling methods, to convert the acid insoluble refractory oxidesto lower state acid soluble forms thereof as by the use of hot hydridebaths or caustic solutions operated at elevated temperatures. Afterconversion, the lower state oxides typically are removed in an acidicsolution such as a nitrichydrofluoric acid bath.

While the prior art descaling methods and solutions have beensatisfactory to some extent there are many disadvantages inherent intheir use for the purpose. The

5 preliminary scale conditioning or conversion method,

for example, is both dangerous and expensive to use. The hot causticbath presents a constant danger to operating personnel through exposureto the caustic by accidental phsyical contact therewith and possibleserious injury from burns. The caustic bath, moreover, to be functional,must be held at the elevated temperature to maintain the liquid stateand this makes the process expensive both because of the energy requiredand the need for replenishment of raw materials making up the bath.

The acid bath for removingthe lower state oxides, according to the priorart methods, has the disad vantage of introducing nascent hydrogen whichis absorbed by titanium and causes 'embrittlement of the metal. Hydrogenembrittlement frequently occurs, for example, in the use of thenitric-hydrofluoric acid bath.

The caustic-acid prior art descale process isalso objectionable in thatthe conversion to the lower state oxides often timesis not uniform andthis permits a socalled preferential attack of the acids on the barebasis metal with resultant step-etching of the articles being descaled.In the hot forming of titanium it is customary, for example, to use someform ofa sprayedon heat barrier to prevent excessive oxidation of thesubstrate metal. When this cover becomes abraded in certain areas, dueto some metal working operation such as may be caused by dies, to thusexpose the substrate metal, the exposedmetal surfaces becomeoxidized toa higher state than those areas protected by the heat barrier cover. Anyattempt to fully descale the higher oxidized areas inevitably produces asteppedetching of the basis metal underlying the lesser oxidized areas,which having been protected by the heat barrier, respond normally to thecaustic-acid descale treatment. The readily descaled areas are thussubjected to an etchant action of the acids on the basis metal duringprolonged immersion and/or reimmersion of the articles as required toremove the higher oxidized areas.

Certain of the aforedescribed difficulties of the prior art caustic-aciddescale process are obviated by use of the electrolytic processdisclosed in US. Pat. No. 2,780,594 to John J. Dailey wherein thetitanium part to be descaled is made the cathode in an electrolytic.system which employs an aqueous acid bath operated at ambienttemperature. The anode in the electrolytic system is either titanium ora suitable ferrous metal, and the direct current is reversed atintervals to effect loosening and removal of the oxide from thesubstrate. Such an electrolytic method has the advantage of avoiding thehigh cost and hazards inherent in the operation of the caustic baths athigh temperatures, but the process is objectionable in that the aqueousacid bath produces copious quantities of nascent hydrogen at the cathodewhich is intolerable from the standpoint of hydrogen embrittlement ofthe part being descaled. Evolution of hydrogen, moreover, evidences anundesired attack on the metal, and this becomes many times greater whenthe current is removed due to the fact that the acids ionize readily inthe highlyaqueous medium. The aqueous electrolytic process and bath ofthe Dailey patent is also objectionable because of the color effectswhich it produces on the surfaces-of the titanium articles descaled inits use. It is well known that titanium will exhibit many differentcolors when subjected to the influence of an. impressed voltage withinthe confines of an aqueous electrolytic medium.

SUMMARY OF THE INVENTION In accordance with the electrolytic descalingmethod and bath of the present invention, the difficulties andlimitations of the prior art methods and baths are largely obviated bymaking the titanium article to be descaled the anode in an electrolyticsystem in which the electrolyte is non-aqueous and provides a smallacidic ion which, under current flow, attacks and penetrates the scaleto forcibly release its tenacious attachment to the substrate metal. Theelectrolytic bath also provides buffer materials for averting pitting ofthe titanium surfaces in the event the current should becomeinadvertently high, and further contains inhibitor materials forpreventing direct acid attack on the metal in the event the current flowis discontinued. To this end, the electrolytic bath compriseshydrofluoric acid which provides the acidic attack ion, concentratedsulfuric acid which serves as the buffer, acetic acid which provides anon-aqueous carrier in lieu of the water carrier of the aqueous bath ofthe Dailey patent, aforesaid, and a mixture of amides and an acetylenicalcohol, which mixture serves as the inhibitor in the bath. In addition,the bath preferably includes an an-ionic wetting agent which will notbreak down in the presence of the highly acidic materials of the bath,and the'surface of the bath preferably is covered with a layer ofmineral oil to suppress fumes emanating from the acetic acid and also toreduce evaporation of the hydrofluoric acid.

In the anodically operated electrolytic descaling process of the presentinvention which is operated at ambient temperature, a cathode formed ofa suitable material such as copper is employed and low current densitiesranging from 0.1 to 5.0 amperes per square foot are used whicheffectively remove refractory oxides in from 2 to 30 minutes, thespecific processing time in a given case being dictated by thetemperature to which the titanium article has been subjected in the hotforming or heat treat operations thereof. Under the influence of theimpressed voltage, the nascent hydrogen collects at the cathode to thusavoid hydrogen pickup and embrittlement of the titanium articlesundergoing the descaling operation. The process, moreover, ischaracterized as one having the property of reducing the amount ofembrittling hydrogen which was present in the titanium article prior tothe removal of the scale therefrom.

The constituents of the highly acid bath have the property of beingmutually miscible whereby the entire bath is formed readily by materialson hand at the descaling site, or the materials at hand may be limitedto the bulky acetic and sulfuric acids to which are added the remainingbath constituents in the form of a purchasable additive.

OBJECTS OF THE INVENTION It is an object of the present inventiontherefor to provide an electrolytic process and bath for descalingarticles formed of titanium and its alloys which may be operated safelyat ambient temperature levels and at low cost and without resultanthydrogen embrittlement, alteration of mechanical properties, or metalloss.

Another object is to provide an anodically operative electrolyticprocess for descaling articles formed of titanium and its alloys.

Another object is to provide .a non-aqueous electrolytic bath fordescaling articles formed of titanium and its alloys-in which a miscibleorganic acid serves as a liquid carrier for the other materials in thebath.

Still another object in an electrolytic descale process and bath asaforedescribed is to prevent acid attack on the articles being descaledin the event either of an increase in the current flow or adiscontinuation thereof.

Yet another object resides in the provision of a descale electrolyticbath as aforedescribed which contains no chlorides to promote stresscorrosion cracking.

Still a further object is to provide an anodically operated electrolyticdescale bath effective to descale titanium and its alloys at low currentdensity levels.

An additional object is to provide an electrolytic descale bath asaforedescribed in which certain constituents of the bath may be packagedand sold as an additive to major constituents of the bath.

Yet another object is to provide an electrolytic descale process inwhich the period of descale is timed according to the thermal exposureof the articles being descaled.

Still other objects, features and advantages of the present inventionwill appear as the description of the preferred embodiment thereofproceeds with reference to specific proportions of the bath ingredientsand specific operative conditions of the bath which give satisfactoryresults.

Titanium is categorized as a reactive" metal owing to its affinity fornon-metallic elements such as oxygen, nitrogen, sulfur and others. Whenexposed to elevated temperatures in the presence of air, titaniumreadily combines with oxygen to produce an oxide of the metal. Thechemical structure of the oxide (scale) is completely dependent for itsformation on the surrounding temperature and the availability of oxygen.The oxides which are formed on the surface of titanium by exposing themetal to temperatures of from 700 to 900F are readily soluble in acid.It is considerably more difficult to remove scale from the metal by acidattack when exposure temperatures fall within the range of l,0001,l50F., and temperatures in excess of l,l50F produce scale so highlyrefractory in composition that acid attack is futile. Other means musttherefore be resorted to for removal of the refractory oxides.

It has been standard practice for many years to treat the metal scale to50 percent aqueous sodium hydroxide at a temperature of 275F eitheralone or in combination with oxidizing agents such as sodium dichromate,potassium permanganate or sodium nitrate. A solution of the concentratedcaustic with a small percentage of copper sulfate is sometimes used toaid in scale removal. This mode of treatment supposedly creates asoftening or conditioning of the oxide to such an extent that it issomewhat more readily removed by acid attack. The acid in mostinstances, as aforementioned, is a combination of .nitric andhydrofluoric acids.

Conditioning is also accomplished by sodium hydride treatment at 700Fand by other processes operative at high temperatures and knowncommercially by the tradenames Kolene and Virgo. Each of the processesis a precursor to the final acid attack for scale removal. Each isextremely hazardous and expensive to operate.

The electrolytic removal of scale from titanium has been practicedheretofore as described in the Dailey patent, aforesaid. The patentstates that efficient removal of scale in an aqueous acidic solution ispossible by a process in which the scale is attacked cathodically. This,however, inevitably leads to interstitial deposition of hydrogen withundesirable embrittlement of the metal. Excessive metal loss is alsoattendant with this process in the event of current failure.

The descale process of the present invention may be used effectively onthe following exemplary materials:

Titanium Titanium (Commercially Pure) Titanium 6 Al 4 V (6 4)Titanium--8All V-l Mo(8-l-l) Titanium -6Al6V-2Sn (6-6-2) Ferrous AlloysStainless Steels 300 Series Precipitation Hardening Steels (l7 7) (PH 157 Mo) lnconel Hastelloy These materials are cleaned prior to thermalexposure by immersion in one of the following baths which are suitablefor the purpose.

Cold Alkaline Cleaner 528 B of Greater Mountain Chemical Company of LosAngeles, Calif.

Hot Alkaline Cleaner 6470A of B & B Chemical Company ofHialeah, Fla.Acid Cleaner WEBCO 1315 of W. E. Brantner Company of San Diego, Calif.

Of these cleaners, the acid cleaner removes all contaminants uponimmersion of the articles therein for a time period of the order ofabout one minute, and its use is preferred. This cleaner is a mixture ofchromic and sulfuric acids and is also used to strip epoxy coatings asdisclosed and claimed in the copending patent applicationof Earl W.Kendall for Process and Composition for Removing Protective Paint Films,Ser. No. 507,671, filed Nov. 15, 1965, now U. S. Pat. No. 3,379,645,issued Apr. 23, 1968.

It has been found in practice and as a result of extensiveexperimentation that the surfaces of titanium must be meticulously cleanbefore exposure to high temperatures. The presence of fingerprints, oil,dirt, mill markings or any contaminant foreign to the metal willcarbonize under elevated thermal conditions and become an integral partof the surface structure. Sub-j sequent removal is extremely difficult.

The cleaned titanium or ferrous articles, as the case may be, may becoated prior to thermal exposure with a spray-applied heat coversuitable for the purpose such, for example, as the commercially knownTurco Pretreat material. Whether protected against oxidation by such aheat barrier material or exposed to the high temperature without theprotective cover, the heated articles after hot forming, or heattreatment as the case may be, are immersed and connected as the anode inthe electrolytic bath of the present invention, this bath having thefollowing formulation of ingredients and proportions.

Electrolytic Bath by wt. by volume Acetic Acid (Glacial) 57 73 55 70Sulfuric Acid (Concentrated) 34 t 20 30 Hydrofluoric Acid (48 50%) 6 910 l5 Inhibitor 0.3 0.5 Wetting Agent 0.2 0.3 Mineral Oil A DC. voltageis impressed across the anode and a suitable cathode, this voltage beingadequate to provide a current density therebetween of the order of 0.1to 5.0 amperes per square foot. After a time period of the order of 2 30minutes, the voltage is removed. The articles are then removed from theelectrolytic bath and the loosened scale (refractory oxides) are simplywashed away in a water rinse.

The cathode may be formed of any one of a group of suitable conductormaterials including copper, aluminum, stainless steel, titanium andcarbon, the copper being preferred when taking into account varioussignificant factors and parameters such as electrical conductivity,cathodic corrosion, and vitiation of bath composition. Aluminum has atendency to corrode in contact with the electrolyte, and the stainlesssteel has a tendency to cause staining of titanium articles after thescale has become softened or removed in the bath. Titanium offers toomuch electrical resistance to be economical, and carbon tends todisintegrate and foul the bath solution.

The bath is non-aqueous and the acetic acid serves as the carrier.Formic acid which is another of a group of.

miscible liquid organic acids may also be used for the purpose, butacetic acid is preferred. In the formation of the bath, the acetic andsulfuric acids which are mixed first, generate considerable'heat whichis believed to be due to the pickup by the sulfuric acid of the smallwater content of the glacial acetic acid. This ionization of thesulfuric acid is believed to aid in the electrolytic process.

The hydrofluoric acid may be supplied by a hydrofluoric acid bearingcompound selected from the group of fluo acids including hydrofluoric,fluoboric and fluosilicic acids.

The hydrofluoric acid, inhibitor and wetting agent preferably are mixedseparately and suitably packaged, as a liquid or in the form of apaste,for sale as an additive for mixing on-site with the initially mixedacetic and sulfuric acids. In either form of the additive, its contentis largely hydrofluoric acid which makes the paste form preferable fromthe standpoint of ease and safety of handling, and for additionalreasons. In the preparation of the paste a suitable thickener such assub-microscopic pyrogenic silica (Cabosil) may be employed for thepurpose.

The sulfuric acidserves as a buffer to limit the metal attack of thehydrofluoric acid on the metallic articles in the bath, particularly inthe event of an inadvertent increase in the current flow. By bufferingis meant that the sulfuric acid acts as a dehydrating agent picking up.water from the hydrofluoric acid (HF) which, in turn, minimizes orprevents ionization of the hydrofluoric acid, which, if not deionized,would cause attack on the titanium; Another buffering agent which mightbe used in an equivalent capacity is acetic anhydride or sulfur trioxide(50;) (the anhydride of sulfuric acid). For such purposes, the aqueouselectrolytic bath consists essentially of a hydrofluoric acid. bearingcompound providing an acidic attack ion in the bath which in response tocurrent flow attacks and penetrates the scale on the article to forciblyrelease its attachment to the substrate metal, and substantiallynon-aqueous glacial acetic acid as the carrier in the bath. Thehydrofluoric acid bearing compound is derived from 48 50 percenthydrofluoric acid which comprisesfi 9 percent by weight of the bath, thebalance being glacial acetic acid.

The inhibitor, on the other hand, limits the hydrofluoric acid attack inthe event the current is inadvertently discontinued. The inhibitor forthis purpose is a mixture of amides and acetylenic alcohol in which theamides contain the general structure of which the alkyl groupapproximates the C range. This inhibitor mixture preferably is composedof 65 percent by volume of the amides and 35 percent by volume ofpropargyl alcohol.

A preferred wetting agent is BENAX 2A1 of Dow Chemical Company, ofMidland, Michigan. This is an anionic surfactant completely effective ina high acidic medium and identified as dodecylated oxydibenzene.disulfonate sodium salt. V

A cover layer of mineral oil such as liquid paraffin of the order ofone-fourth to 1 inch thick is maintained at the surface of the bath. Theoil cover does not enter into any of the reactions of the electrolyticbath medium. The cover, however, serves effectively to reduce thevolatilization of the hydrofluoric acid and to Brightener by weightAmmonium Bifluoride 0.l 0.2 Citric Acid 1.5 3.0 Ammonium Persulfate 4.08.0 Water (Deionized) Balance The fluoride ion attacks the metalarticles, the attack, however, being buffered somewhat by the citricacid as disclosed in U.S. Pat. No. 3,003,896 issued to Earl W. Kendallon Oct. 10, 1961. The ammonium persulfate addition to the patentedammonium bifluoride citric acid composition serves to inhibit the attackof the fluoride ion while permitting a passivation and oxidation actionin the brightener which produces the desired lustre and metallicappearance.

CURRENT DENSITIES Optimum clean surface conditions of the descaledarticles are obtained when anode current densities are employed inaccordance with the thermal exposure of the articles. A mottled surfacecondition results, for example, when the anode current density is heldconstant at 5 amperes per square foot regardless of the thermal exposureconditions. The desired variation of the anode current density toproduce a clean surface on an exemplary specimen for different exposuretemperatures is given in the following table.

Temperature Anode Current Density Degrees F Amperes per square foot l2001.0

SURFACE CLEANLINESS Three panels of Titanium 8A1 lV 1M0 were preparedfor test. Panel No. l was used as received with no preparatorytreatment. Panel No. 2 was cleaned in an alkaline medium for 10 minutes.Panel No. 3 was cleaned in the aforesaid acid cleaner for 1 minute. Allthree panels were protected by Turco Pretreat and then exposed to heatat l,100F for minutes. After descaling by the hereinbefore describedelectrolytic process and bath of the'present invention, followed bywater rinsing, and immersion for 3 minutes in the aforedisclosedbrightener, the panels were inspected for cleanliness.

Panel No. 1 clearly evidenced a carbonizing and burning" thereinto offingerprints, oil and other contaminants. Panel No. 2 which evidenced awaterbreak free" surface prior to heat exposure and application of theTurco Pretreat, when observed after descaling, evidenced the presence ofcarbonized contaminants, thereby suggesting that the so-calledwaterbreak free surface test is a fallacious indication of surfacecleanliness and cannot be completely relied upon. Panel No. 3 evidencedno trace of carbonized materials, thereby attesting to the effectivenessof the acid cleaner in completely removing all surface contaminants.

RATE OF SCALE REMOVAL Sixteen specimen panels of titanium 8Al lV lMo, 10X 3 X 0.20 inch, were cleaned in a cold alkaline cleaner. The panelswere then divided into two groups of eight panels each and thermallyexposed. One group was numbered 10 through 17 and the other 10T.

through l7T to designate by the added letter T that the panels of thesecond group prior to thermal exposure had each been protected by anapproximately 0.6 mil thick layer of Turco Pretreat, the aforementionedspray-applied heat cover. Each number of each group represented thetemperature of exposure in hundreds of degrees Fahrenheit to which thepanel had been exposed. Panels 10 17 were not protected by a heat cover.Using an anode to cathode separation of 1 inch and a current density ofl ampere per square foot, the panels were then descaled in theelectrolytic bath of the present invention, followed by a water rinseand immersion in the brightener, to disclose comparatively in terms oftime of descale required, the ease of effecting scale removal in the useof a heat barrier. The results of the tests tabulated below disclosethat the presence of an oxidation-resistant barrier on the surface oftitanium is beneficial from the standpoint of reducing the time requiredfor scale removal when the metal is exposed to temperatures upwards ofl,300F.

Panel Exposure Temperature Descale Time No. Degrees F Minutes 10 1000 210T 2 l l l 3 l tT 3 I2 1200 5 l2T 5 13 1300 5 131 5 14 1400 20 l 4T 1015 1500 25 1ST l5 16 1600 35 16T l5 17 1700 40 [7T l5 HYDROGENEMBRITTLEMENT Specimen panels 10 X 2 X 0.020 inch of titanium 8A1 lV 1M0were-stamped respectively with the numbers l 8, each panel being stampedat both ends with the same number. All panels were thoroughly cleaned inan alkaline medium, and a 1.2 inch numbered strip then removed from theend of each of the panels and analyzed for hydrogen content, thesestrips being referred to hereinafter as the as received. specimens.

The surfaces of the remaining portions of the 8 panels were then coatedwith Turco Pretreat and exposed to heat at 1,450F for a period of 15minutes. All panels were descaled by the process of the presentinvention using a current density of 4.5 amperes per square foot, andthereafter, following rinsing, were treated in the brightener bath for 3minutes. After descaling, a 56 inch numbered test strip was removed fromeach of the descaled panels and analyzed for hydrogen content, thesetest strips being referred to hereinafter as the descaled" specimens.

The non-embrittling properties of the hereindisclosed anodicallyfunctional electrolytic bath is disclosed in the table set forthhereinbelow wherein the hydrogen content of the as received anddescaled, numbered strip specimens are compared.

In the table, the as received specimens are numbered 1 to 8, and thedescaled specimens are numbered 1D to 8D.

Hydrogen Content Specimen Parts Per Million 1 63 1D 60 2 62 2D 59 3 633D 6| 4 65 4D 65 5 69 5D 67 6 64 6D 66 7 73 7D 51 8 67 8D 52 descaled bythe aforementioned prior art caustic-acid descale process involvingscale conditioning by immersion in an aqueous 50 percent sodiumhydroxide solution operating at 275F followed by scale removal in anitric-hydrofluoric acid bath. A second 14 panel set of panels identicalwith the first set and numbered 1K 14K (Kendall), was descaled by theelectrolytic process of the present invention (the Kendall process). Thehydrogen content of the panels of each set was determined followingdescale.

For the purpose .of these tests, four panels of commercially puretitanium (C.P.), four panels of Titanium 6A1 6V 2Sn (6-6-2), and sixpanels of Titanium 6A1 4V (6-4), were used in each set of 14 panels andexposed to temperatures of 1,200", 1,300, l ,500, l,600 and 1,700F.Prior to thermal exposure, the panels were deliberately vitiated withfingerprints, grease, and shop soil. No cleaning of any nature of thepanels was attempted. In each set of 14 panels two panels of puretitanium, two panels of'6-6-2 alloys, and three panels of the 6-4alloywere each coatedwith a forming lubricant hereinafter designatedE andknown commercially as Everl'ube T50, and two panels of pure titanium,two panels of 6-6-2 alloy, and three panels of 6-4 alloy were eachcoated with the aforesaid Turco Pretreat herein designated T.

The results of these tests are given in the following table from whichit is manifest that the specimens descaled by the herein disclosed andclaimed Kendall Titanium Coating Exposure Hydrogen Content CompositionTemp. parts per million( ppm) Caustic degrees F Acid Kendall DifferenceProcess Process panel ppm panel ppm C.P. E 1200 58 1K 48 10 C.P. E 13002 49 2K 42 7 CF. T. 1200 3 47 3K 44 3 CF. --T 1300 4 43 4K 42 1 6-6-2 E1300 5 109 5K 69 40 6-6-2 E 1600 6 6X 92 3 6-6-2 T 1300 7 123 7K 72 516-6-2 T. 1600 8 122 8K 104 18 6-4 B 1300 9 97 9K 73 24 6-4 B 1500 10 10210K 83 -l 9 6-4 E 1700 11 11K 148 -37 6-4 T 1300 12 96 12K 71 25 6-4 T1500 13 98 13K 86 12 6-4 T 1700 14 123 14K 77 46 ETCH RATE(ELECTROLYTIC) A series of tests were made at various current densitylevels to determine the rateof attack of the herein disclosed anodicallyoperatedKendall descale bath on the substrate metal. Six panels oftitanium 6A1 4V, 3 X 4 inches, were thoroughly cleaned in a coldalkaline medium, rinsed in deionized water, and dried. All panels wereidentified by metal stamping (l 6) and weighed to the fourth decimalplace. These panels were subjected to the following six current densitylevels of 0.43, 0.48, 0.80, 1.38, 1.60 and 1.80 amperes per square footfor periods of time extending from 10 to 25 minutes. The panels werethen weighed and compared with the initial weight measurements, theresults being given in the following table.

Electrolytic Etch Loss and Rate Current Panel Density Weight in GramsEtch No. amp/ft Initial Final Loss time Rate min. mg/ftl min I 0.4326.0087 26.0055 .0032 20 .77 2 0.48 26.5285 26.5248 .0037 20 .89 3 0.8043.9417 43.9320 .0097 25 t 3.1 4 1.38 43.7994 43.7856 .0138 10 11.0 51.60 44.1420 44.1212 .0208 10 16.6 6 1.80 15.1207 15.0776 .0431 10 25.9

ETCH RATE (CHEMICAL) Two panels of titanium 6A1 4V, 1 X2 inches, werethoroughly cleaned and weighed to the fourth decimal place. Both panelswere'then completely submerged in the electrolytic bath of the presentinvention for 24 hours and without currentflow. After'rinsing anddrying, the panels were again weighed to the fourth decimal place.and'the losses calculated. The results are given in the following tablefrom which it may be noted that the loss due to chemical etch is 0.0063gram which, calculated from the square foot area and etch time employed,corresponds to an etch rate of 9.5 milligramsper square foot per hour(mgJft'lhr).

Chemical Etch Loss and Rate average 4.2731 4.2668 0.0063 9.5

It will be noted that both the electrolytic and chemical tests were madeusing specimens of titanium 6A1- 4V, and it will further be noted thatthe electrolytic etch rate of 0.77 mglft lmin for the lowest currentdensity employed corresponds to 46.2 mglft /hr which is considerablygreater than the calculated chemical etch rate of 9.5 mg/ft lhr, thisbeing due to the enhanced ionization accompanying the current flow. Thechemical etch rate, on the other hand is low due in part to thenon-aqueous nature of the electrolytic bath and the buffering action ofthe sulfuric acid and the inhibiting action of the amides and acetylenicalcohol.

BEND TESTS Ten panels of titanium 8A1 1V 1M0, 4 X 2% X 0.016 inch, werecleaned and prepared for bend tests. Five of the ten panels were left inthe as cleaned" condition, and the other five were coated with TurcoPretreat and exposed to heat at 1,450F forlS minutes. Subsequent to thisexposure, the five panels were descaled in the hereindisclosedelectrolytic bath and brightener. The five as cleaned" and the fivedescaled panels were then subjected to ST and 1.5T bends which wereaccomplished equally well by both sets, thereby indicating comparativelythat no adverse structural effects are introduced by the descaleprocess.

TENSILE TESTS Five tensile specimens of titanium 8A1 1V 1M0, 0.020 gage,were duplex annealedat 1,450F. After cooling to ambient temperature of70 75F, the specimens were descaled in the electrolytic process of thepresent invention including the brightener. Mechanical properties ofultimate yield and percent elongation were subsequently determined andtabulated in the following table from which it may be ascertained byreference to Military Specifications MIL-T-9 046 that the mechanicalproperties arenot adversely affected by the electrolytic descaleprocess.

Mechanical Properties Specimen Ultimate Load Yield Point Elongation Nolbs. 0.2% lbs 2 inch 1 145,000 132,000 14.0 2 145,300 134,000 13.0 3147,000 133,500 14.6 4 148,500 134,200 12.0 5 145,000 132,200 12.3 Avg146,100 133,200 13.2

The desirability of precleaning titanium surfaces which are to bedescaled subsequent to thermal exposure has been emphasized herein inorder to provide a highly efficient and economical industrial process.It will be understood, however, that the hereindisclosed anodicallyoperated electrolytic descale bath is effective to produce cleansurfaces in the presence of severe contamination such as fingerprintsand shop soil, oxidation barriers such as Turco Pretreat, and/or forminglubricants such as Everlube T50. This was the condition deliberatelyestablished in the aforedescribed series of tests for determininghydrogen content; While the surfaces of the descaled panels wereacceptably clean, a greater period of current flow in the electrolyticprocess was required to remove the evidences of contamination.

The novel principles of this invention transcend the scope of theinvention as suggested or implied by the foregoing disclosure thereof,and the invention may be embodied in other forms or carried out in otherways which have been conceived and reduced to practice during the courseof this development, without departing from the spirit or essentialcharacteristics of the invention. The invention disclosed hereintherefore is to be considered as in all respects illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims, and all changes which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

Having thus described the invention, what is claimed as new and usefuland desired to be secured by Letters Patent is:

1. An additive to concentrated sulfuric acid dissolved in a non-aqueousliquid organic acid as the carrier therefor to form an electrolytic bathfor descaling an article formed of titanium and its alloys comprising ahydrofluoric-acid bearing compound, an anionic wetting agent, and aninhibitor for inhibiting attack of the hydrofluoric acid on the articleswhen current flow in the bath is discontinued, said inhibitor consistingof a mixture of amides and an acetylenic alcohol, said amides containingthe general structure in which the alkyl group approximates the C, rangeand wherein the amides and the acetylenic alcohol respectively are 65and 35 percent by volume of the inhibitor, said wetting agent being ananionic surfactant in the form of a dodecylated oxydibenzene disulfonatesodium salt, said carrier being in amount equivalent to 55 to percentglacial acetic acid by volume of the bath, said sulfuric acidconstituting 20 to 30 percent by volume of the bath, said hydrofluoricacid bearing compound being in amount equivalent to 10 to 15 percenthydrofluoric acid (48-50 percent) by volume of the bath, said inhibitorconstituting 0.3 to 0.5 percent by volume of the bath, and said wettingagent constituting 0.2 to 0.3 percent by volume of the bath.

. 2. An additive as in claim 1 and wherein the carrier is selected fromthe group of liquid organic acids including acetic and formic acids.

3. An additive as in claim 1 wherein the hydrofluoricacid bearingcompound is selected from the group of fluo acids includinghydrofluoric, fluoboric, and fluosilicic acids.

4. A non-aqueous electrolytic bath composition for descaling an articleformed of titanium and its alloys comprising glacial acetic acid,concentrated sulfuric acid dissolved in the acetic acid, andhydrofluoric acid (48-50), said acetic acid comprising from 55 to 70percent by volume of the bath, said sulfuric acid comprising from 20-30percent by volume of the bath, and the hydrofluoric acid being 48 to 50percent acid and comprising 10 to 15 percent by volume of the bath.

5. A composition as in claim 4 and further comprising a non-reactiveinhibitor for inhibiting attack of the hydrofluoric acid on the titaniumarticle when current flow in the bath is discontinued, said inhibitorconsistin which the alkyl group approximates the C, range and whereinthe amides and the [acetylenic alcohol respectively are 65 and 35percent by volume of the inhibitor, said inhibitor comprising from 0.3to 0.5 percent by volume of the bath.

6. A composition as in claim 4 and further comprising an anionic wettingagent operable in the acid bath and having the form of a dodecylatedoxydibenzene disulfonate sodium salt, said wetting agent comprising from0.2 to 0.3 percent by volume of the bath.

7. A composition as in claim 4 and further comprising a layer ofparaffin oil on the acidic bath to prevent loss by vaporization of thehydrofluoric acid and to contain fumes from the acetic acid.

8. A non-aqueous electrolytic bath composition for descaling an articleformed of titanium and its alloys and consisting of the followingconstituents and proportions thereof by volume of the bath:

Acetic Acid (Glacial) said inhibitor consisting of a mixture of amidesand an acetylenic alcohol, said amides containing the general structurein which the alkyl group approximates the C range and wherein the amidesand the acetylenic alcohol respectively are 65 and 35 percent by volumeof the in hibitor, said wetting agent being an anionic surfactant in theform of a dodecylated oxydibenzene disulfonate sodium salt, the sulfuricacid being dissolved in the acetic acid.

9. A non-aqueous electrolytic bath for descaling an article formed oftitanium and its alloys wherein the article is the anode in the bath,said bath consisting essentially of a hydrofluoric acid bearing compoundproviding an acidic attack ion in the bath which in response to currentflow attacks and penetrates the scale on the article to forcibly releaseits attachment to the substrate metal, a buffering agent which acts as adehydrating agent for picking up water from hydrofluoric acid

2. An additive as in claim 1 and wherein the carrier is selected fromthe group of liquid organic acids including acetic and formic acids. 3.An additive as in claim 1 wherein the hydrofluoric-acid bearing compoundis selected from the group of fluo acids including hydrofluoric,fluoboric, and fluosilicic acids.
 4. A non-aqueous electrolytic bathcomposition for descaling an article formed of titanium and its alloyscomprising glacial acetic acid, concentrated sulfuric acid dissolved inthe acetic acid, and hydrofluoric acid (48-50), said acetic acidcomprising from 55 to 70 percent by volume of the bath, said sulfuricacid comprising from 20-30 percent by volume of the bath, and thehydrofluoric acid being 48 to 50 percent acid and comprising 10 to 15percent by volume of the bath.
 5. A composition as in claim 4 andfurther comprising a non-reactive inhibitor for inhibiting attack of thehydrofluoric acid on the titanium article when current flow in the bathis discontinued, said inhibitor consisting of a mixture of amides and anacetylenic alcohol, said amides containing the general structure inwhich the alkyl group approximates the C12 14 range and wherein theamides and the acetylenic alcohol respectively are 65 and 35 percent byvolume of the inhibitor, said inhibitor comprising from 0.3 to 0.5percent by volume of the bath.
 6. A composition as in claim 4 andfurther comprising an anionic wetting agent operable in the acid bathand having the form of a dodecylated oxydibenzene disulfonate sodiumsalt, said wetting agent comprising from 0.2 to 0.3 percent by volume ofthe bath.
 7. A composition as in claim 4 and further comprising a layerof paraffin oil on the acidic bath to prevent loss by vaporization ofthe hydrofluoric acid and to contain fumes from the acetic acid.
 8. Anon-aqueous electrolytic bath composition for descaling an articleformed of titanium and its alloys and consisting of the followingconstituents and proportions thereof by volume of the bath: Acetic Acid(Glacial) 55 - 70% Sulfuric Acid (Concentrated) 20 30% Hydrofluoric Acid(48 - 50%) 10 - 15% Inhibitor (Amide-acetylenic alcohol mixture) 0.3 -0.5% Wetting Agent (Anionic surfactant) 0.2 - 0.3% said inhibitorconsisting of a mixture of amides and an acetylenic alcohol, said amidescontaining the general structure in which the alkyl group approximatesthe C12 14 range and wherein thE amides and the acetylenic alcoholrespectively are 65 and 35 percent by volume of the inhibitor, saidwetting agent being an anionic surfactant in the form of a dodecylatedoxydibenzene disulfonate sodium salt, the sulfuric acid being dissolvedin the acetic acid.
 9. A non-aqueous electrolytic bath for descaling anarticle formed of titanium and its alloys wherein the article is theanode in the bath, said bath consisting essentially of a hydrofluoricacid bearing compound providing an acidic attack ion in the bath whichin response to current flow attacks and penetrates the scale on thearticle to forcibly release its attachment to the substrate metal, abuffering agent which acts as a dehydrating agent for picking up waterfrom hydrofluoric acid formed in the bath to deionize the same andprevent attack on the titanium, and a non-aqueous liquid organic acid asthe carrier in the bath, said hydrofluoric acid bearing compound beingin amount equivalent to 10 to 15 percent hydrofluoric acid by volume ofthe bath, said carrier being in amount equivalent to 55 to 70 percentglacial acetic acid by volume of the bath, and said buffering agentbeing in amount equal to 20 to 30 percent concentrated sulfuric acid byvolume of the bath.